Downlink Synchronization Method, and Apparatus and System Cross-Reference to Related Applications

1. A communications method, comprising: configuring, by a first base station, carrier aggregation or dual connection on a first cell and a second cell for a terminal; sending, by the first base station, downlink synchronization reference information to the terminal by using dedicated signaling or a system message, the downlink synchronization reference information for instructing the terminal to perform downlink synchronization in the second cell by applying a relationship between a time domain source of a second downlink synchronization channel of the second cell and a time domain source of a first downlink synchronization channel of the first cell, wherein the second cell is on a second carrier and the first cell is on a first carrier, wherein a second downlink synchronization channel of the second cell is detected by the terminal according to the downlink synchronization reference information.

2. The method according to claim 1 , further comprising: sending, by a second base station and in the second cell, the second downlink synchronization channel to the terminal, wherein the second base station is the first base station or a base station other than the first base station.

3. The method according to claim 2 , wherein the method further comprises: determining, by the first base station, that the terminal needs to perform downlink synchronization in the second cell.

4. The method according to claim 2 , wherein after sending, by the second base station in the second cell, the second downlink synchronization channel to the terminal, the method further comprises: receiving, by the first base station, a detection result of the second downlink synchronization channel of the second cell that is reported from the terminal; and determining, by the first base station, according to the detection result, whether to configure the second cell as a serving cell of the terminal.

5. The method according to claim 2 , wherein sending, by the second base station in the second cell, the second downlink synchronization channel to the terminal comprises: sending, by the second base station at any orthogonal frequency division multiplexing (OFDM) symbol location in a subframe in the second cell, the second downlink synchronization channel to the terminal.

6. The method according to claim 5 , wherein each OFDM symbol comprises beams in at least two directions, and all the beams in the at least two directions use different radio frequency (RF) chains.

7. The method according to claim 2 , wherein sending, by the second base station in the second cell, the second downlink synchronization channel to the terminal comprises: sending, by the second base station at a fixed orthogonal frequency division multiplexing (OFDM) symbol location in a subframe in the second cell, the second downlink synchronization channel to the terminal.

8. The method according to claim 1 , wherein the second cell is a neighbor cell of the first cell.

9. A communications method, comprising: receiving, by a terminal, downlink synchronization reference information from a base station by using dedicated signaling or a system message, wherein the downlink synchronization reference information instructs the terminal to perform downlink synchronization in a second cell by applying a relationship between a time domain source of a second downlink synchronization channel of the second cell and a time domain source of a first downlink synchronization channel of the first cell, wherein the second cell is on a second carrier and the first cell is on a first carrier; and detecting, by the terminal according to the downlink synchronization reference information, a second downlink synchronization channel of the second cell.

10. The method according to claim 9 , wherein after detecting, by the terminal according to the downlink synchronization reference information, the second downlink synchronization channel of the second cell, the method further comprises: reporting, by the terminal, a detection result of the second downlink synchronization channel of the second cell.

11. The method according to claim 9 , wherein detecting, by the terminal according to the downlink synchronization reference information, the second downlink synchronization channel of the second cell comprises: detecting, by the terminal according to information about the first downlink synchronization channel of the first cell, the second downlink synchronization channel of the second cell.

12. The method according to claim 9 , further comprising: receiving, by the terminal, a configuration about carrier aggregation or dual connection on the first cell and the second cell for the terminal.

13. The method according to claim 9 , wherein the second cell is a neighbor cell of the first cell.

14. An apparatus, comprising: a receiving circuit to receive downlink synchronization reference information from a base station by using dedicated signaling or a system message, wherein the downlink synchronization reference information instructs the apparatus to perform downlink synchronization in a second cell by applying a relationship between a time domain source of a second downlink synchronization channel of the second cell and a time domain source of a first downlink synchronization channel of the first cell, wherein second cell is on a second carrier and the first cell is on a first carrier; and a processor to detect, according to the downlink synchronization reference information received by the receiving circuit, a second downlink synchronization channel of the second cell.

15. The apparatus according to claim 14 , further comprising: a transmitting circuit to report a detection result of the second downlink synchronization channel of the second cell.

16. The apparatus according to claim 14 , wherein the processor is to detect, according to information about the first downlink synchronization channel of the first cell, the second downlink synchronization channel of the second cell.

17. The apparatus according to claim 14 , wherein the receiving circuit is further configured to receive a configuration about carrier aggregation or dual connection on the first cell and the second cell for the terminal.

18. The apparatus according to claim 14 , wherein the second cell is a neighbor cell of the first cell.